The coccyx is formed of either three, four or five rudimentary vertebrae. It articulates superiorly with the sacrum. In each of the first three segments may be traced a rudimentary body and articular and transverse processes; the last piece (sometimes the third) is a mere nodule of bone. The transverse processes are most prominent and noticeable on the first coccygeal segment. All the segments lack pedicles, laminae and spinous processes. The first is the largest; it resembles the lowest sacral vertebra, and often exists as a separate piece; the remaining ones diminish in size from above downward.

Most anatomy books incorrectly state that the coccyx is normally fused in adults. In fact it has been shown that the coccyx may consist of up to five separate bony segments, the most common configuration being two or three segments.[1][2]

The anterior surface is slightly concave and marked with three transverse grooves that indicate the junctions of the different segments. It gives attachment to the anterior sacrococcygeal ligament and the levatores ani and supports part of the rectum. The posterior surface is convex, marked by transverse grooves similar to those on the anterior surface, and presents on either side a linear row of tubercles–the rudimentary articular processes of the coccygeal vertebrae. Of these, the superior pair are the largest, and are called the coccygeal cornua they project upward, and articulate with the cornua of the sacrum, and on either side complete the foramen for the transmission of the posterior division of the fifth sacral nerve.

The lateral borders are thin and exhibit a series of small eminences, which represent the transverse processes of the coccygeal vertebrae. Of these, the first is the largest; it is flattened from before backward, and often ascends to join the lower part of the thin lateral edge of the sacrum, thus completing the foramen for the transmission of the anterior division of the fifth sacral nerve; the others diminish in size from above downward, and are often wanting. The borders of the coccyx are narrow, and give attachment on either side to the sacrotuberous and sacrospinous ligaments, to the coccygeus in front of the ligaments, and to the gluteus maximus behind them.

The extensor coccygis is a slender muscle fascicle, which is not always present. It extends over the lower part of the posterior surface of the sacrum and coccyx. It arises by tendinous fibers from the last segment of the sacrum, or first piece of the coccyx, and passes downward to be inserted into the lower part of the coccyx. It is a rudiment of the extensor muscle of the caudal vertebrae of other animals.

In humans and other tailless primates (e.g., great apes) since Nacholapithecus (a Miocenehominoid),[5][6] the coccyx is the remnant of a vestigial tail, but still not entirely useless;[7] it is an important attachment for various muscles, tendons and ligaments—which makes it necessary for physicians and patients to pay special attention to these attachments when considering surgical removal of the coccyx.[8] Additionally, it is also a part of the weight-bearing tripod structure which acts as a support for a sitting person. When a person sits leaning forward, the ischial tuberosities and inferior rami of the ischium take most of the weight, but as the sitting person leans backward, more weight is transferred to the coccyx.[8]

Injuring the coccyx can give rise to a painful condition called coccydynia and one or more of the bones or the connections thereof may be broken, fractured tailbone.[10][11] A number of tumors are known to involve the coccyx; of these, the most common is sacrococcygeal teratoma. Both coccydynia and coccygeal tumors may require surgical removal of the coccyx (coccygectomy). One very rare complication of coccygectomy is a type of perineal hernia known as a coccygeal hernia.[12]

This established etymological explanation can also be found in the writings of the 16th century anatomist Andreas Vesalius whom wrote: os cuculi, a similitudine rostri cuculi avis[16] (the cuckoo bone shows a likeness to the beak of the cuckoo bird). Vesalius used the Latin expression os cuculi, with os, bone[20] and cuculus, the Latin name for the cuckoo.[20] The 16th/17th century French anatomist Jean Riolan the Younger gives a rather hilarious etymological explanation, as he writes: quia crepitus, qui per sedimentum exeunt, ad is os allisi, cuculi vocis similitudinem effingunt[16] (because the sound of the farts that leave the anus and dash against this bone, shows a likeness to the call of the cuckoo). The latter is not considered as potential candidate.[16][17]

^Note: Nacholapithecus and Nakaliphitecus nakayamai are two different species of Miocene hominoids (specimens from Nakali and Nachola respectively). See for example "Comparisons with Other Hominoids" in (Kunimatsu, Nakatsukasa et al. Dec 2007)

^ abKlein, E. (1971). A comprehensive etymological dictionary of the English language. Dealing with the origin of words and their sense development thus illustration the history of civilization and culture. Amsterdam: Elsevier Science B.V.